Anaerobic digestion of organic material, such as sewage sludge, municipal waste, animal waste, industrial waste, forestry waste, agricultural waste, and other highly organic carbonaceous material, is the fermentation of such material by bacteria in the absence of oxygen. There are many benefits to anaerobic digestion of organic waste material, including stabilization of the waste, odor and solids reduction, energy production in the form of gas (primarily methane) elimination or reduction of pathogens in waste materials, production of stable, generally environmentally acceptable slurry or sludge which can be used as fertilizer, soil conditioner, or nutrient source, such as a feed supplement and the like. Anaerobic digestion is becoming increasingly important in agricultural arenas, as increasing production of intensive and large-scale production of food animals and animal products has generated an enormous waste disposal problem for the animal industry.
Typically, in conventional methanation treatments of waste, waste is treated at ambient temperatures or at mesophilic temperatures, i.e., body temperatures of about 30.degree. to 40.degree. C., to produce methane gas. For example, U.S. Pat. No. 4,696,746 to Ghosh et al. discloses a two-phase anaerobic digestion process with two discrete methane phase digesters operating in parallel. A source of organic feed material is delivered to an acid phase digester to degrade the organic material to volatile fatty acids or aldehydes or alcohol intermediates. Supernatant from the liquid digester contents of the acid phase digester is conveyed through a liquid conduit to a methane phase I digester. The gaseous product of the acid phase digester is conveyed through a gas conduit to a methane phase II digester. Fermentation is conducted in the methane phase I or II digester under mesophilic or thermophilic temperatures.
PCT Application No. WO 84/00038 describes a process for producing methane gas in which manure is subjected to mesophilic digestion in a first tank and afterwards to thermophilic digestion in a second tank.
U.S. Pat. No. 4,252,901 to Fisher et al. discloses a multi-stage anaerobic digestion process which includes a plurality of digestion modules, each heated by conventional means to a desired temperature.
Despite these and other processes for the anaerobic digestion of organic waste material, it would be advantageous to provide a process and apparatus for anaerobic digestion of organic material which offers simplicity in design, construction and operation and which is highly efficient in biodegradation and the production of methane gas. Further it would be advantageous to provide a process and apparatus which maximizes the energy efficiency of the digestion process. In addition, it would be advantageous to provide such a process and apparatus which provides pathogen control, and provides by-products for economic return.